But after a day at the park last December, the energetic hunting dog quickly became sick and died by the next morning.

While the incident was unfortunate, Testa didn’t think to alert police or public officials until a recent spate of reports about dogs falling ill.

Though he doesn’t know what ultimately left his dog, a Vizsla, dead, a couple of mysterious dog deaths have some residents worried about what could be happening in the county-owned park, including what Testa suspects to be potential poisoning. "My guy was as healthy as a bull," he said. "That was the only thing that could have put him down."

Now, after a second, seemingly related incident at the reservation this past weekend, the Essex County Sheriff’s Office is investigating.

Diane Herbst, of Montclair, said her four-year-old dog Gingee died two hours after leaving the reservation Saturday, March 15. After their second trip to the park, her dog appeared tired and later had trouble walking.

Her veterinarian said it was likely Gingee ingested poison or, less likely, had a blood clot, Herbst said, but she couldn’t yet be sure exactly what happened. (Herbst is an employee of North Jersey Media Group, which publishes the Verona-Cedar Grove Times.)

"I think for dogs to be dying as quick as they’re dying, somebody’s got to be dropping off a very toxic compound," Testa said.

Still, a veterinarian looking into the recent incident warned that panic could be premature. George Cameron of the Cameron Animal Hospital in Montclair said there was only one other recent incident of an animal dying after going to Mills Reservation and that was from eating sticks and stones off the ground. Cameron said Gingee’s cause of death was unknown and called it "presumptive" to say the terrier was poisoned.

"I’ve been here a long time. I haven’t seen any evidence of dogs being poisoned over there," Cameron said. He added that he’s spoken to another Montclair vet and hasn’t found an indication of poisoning.

Herbst said she plans to have her dog autopsied.

Officials from the Essex County Sheriff’s Office and the county’s parks department met with Herbst Monday at the reservation, according to county Spokesman Anthony Puglisi.

"This is the first incident that we’ve heard of this nature," Puglisi said, adding that chemicals aren’t used at Mills Reservation. "It’s serious and we want to look into it and we want to find out what caused it."

Sheriff’s Department Spokesman Kevin Lynch said investigators were waiting to hear back on the results of Gingee’s autopsy, but that was the only case reported to the department thus far.

The 157-acre park spans Cedar Grove and Montclair and is a popular spot for bird watching and dog walking.

According to Testa and Herbst, their dogs had free range in the reservation.

A person who answered the phone at the Cedar Grove Animal Hospital said she’d heard a patient mention the recent incidents, but hasn’t encountered any poisoned pets there.

Another person at Well Pet Animal Hospital in Verona also said there hadn’t been similar incidents matching the description provided.

Still, Testa said it was suspicious for his young dog to be so suddenly stricken, especially given some of the "strange characters" who he has sometimes seen walking through the park.

"For all the weirdness that’s going on in the world today, that shouldn’t take anybody by surprise," he said.